The present invention relates generally to the field of fluorine separation and fluorine generation devices, and more particularly to a novel electrolytic device having unusual and unexpected electrochemical performance.
The semiconductor industry makes extensive use of gas mixtures that contain fluorine. Many of these substances are harmful and expensive, and must therefore be removed or scrubbed from the exhaust gas stream. Accordingly a need exists for a device and method to separate fluorine from gas mixtures, and to generate the fluorine needed for the industry.
There are several approaches to separate fluorine (F2) from other gases, including cryogenic distillation, permeation membranes, and electrolytic separation. Electrolytic separation offers the potential advantages of producing high purity fluorine at room temperature at high flux in a compact unit. Methods are known for the electrochemical separation of gas mixtures. One technique, described in U.S. Pat. No. 5,618,405 teaches the separation of halides from high temperature gas mixtures using an electrochemical cell, the contents of which are hereby incorporated by reference in its entirety. Another technique known as the “outer-cell” method the gaseous component of a waste gas to be stripped are first absorbed in an absorption column in a wash solution; then the wash solution containing the polluting component is cathodically reduced or anodically oxidized in a connected electrolysis cell. This arrangement requires two different devices, namely one for the absorption and one for the electrolysis. Another technique is the “inner-cell” method in which absorption and electrochemical conversion take place in an electrolysis cell, and because the concentration of pollutants is always kept low by electrochemical conversion. Yet another method is the “indirect” electrolysis processes where the oxidizing or reducing agent used in a wet-chemical waste-gas treatment is regenerated by electrolysis of the wash solution used.
U.S. Pat. Nos. 6,071,401 and 5,840,174, the contents of which are incorporated herein by reference in their entirety disclose an electrolysis cell with a fixed-bed electrode for the purification of waste gases. In reductive purification hydrogen is supplied to the gas diffusion electrode and in oxidative purification oxygen is used.
U.S. Pat. No. 6,030,591, the contents of which are incorporated herein by reference in their entirety discloses the separation of fluorocompounds by cryogenic processing, membrane separation and/or adsorption.
U.S. Pat. Nos. 6,514,314 and 5,820,655, the contents of which are incorporated herein by reference in their entirety disclose a ceramic membrane structure an oxygen separation method.
One disadvantage of the inner cell method is the high residual content of impurities in the purified gas. In the case of chlorine the residual content is approximately a factor of ten above the limit value of 5 ppm. In general, the purity of gases generated by solid state devices is much higher than that of liquid (or melt) containing cells.
Another disadvantage of the prior art methods is the fact that the apparatus comprising an electrolysis cell requires two liquid circuits, namely a cathode circuit and an anode circuit, as a result of which the device is rendered complicated and trouble-prone.
Solid-state electrochemical devices are often implemented as cells including two porous electrodes, the anode and the cathode, and a dense solid electrolyte and/or membrane, which separate the electrodes. For the purposes of this application, unless otherwise explicit or clear from the context in which it is used, the term “electrolyte” should be understood to include solid oxide membranes used in electrochemical devices, whether or not potential is applied or developed across them during operation of the device. In many implementations the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as fluorine ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane may be composed of a mixed ionic electronic conducting material (“MIEC”). In each case, the electrolyte/membrane must be dense and as pinhole free as possible (“gas-tight”) to prevent mixing of the electrochemical reactants. In all of these devices a lower total internal resistance of the cell improves performance.
Solid-state electrochemical devices are typically based on electrochemical cells with ceramic electrodes and electrolytes and have two basic designs: tubular and planar. Tubular designs have traditionally been more easily implemented than planar designs, and thus have been proposed for commercial applications. However, tubular designs provide less power density than planar designs due to their inherently relatively long current path that results in substantial resistive power loss. Planar designs are theoretically more efficient than tubular designs, but are generally recognized as having significant safety and reliability issues due to the complexity of sealing and manifolding a planar stack.